We present an observation of quantum interference between position and momentum with photons. We prepared a superposition of position and momentum states by using slits, lenses, and an interferometer. The precise control and measuring of propagation time of the photon results in a clear quantum interference of $85\%$ visibility. The generated quantum state is then used to verify the violation of particle propagation inequality proposed by Hofmann [Phys. Rev. A 96, 020101(R) (2017)], resulting in $5.9\%$ below the statistically predicted lower bound. We believe that this result is a major step towards further developments in quantum theory, such as fundamental modifications to the interpretation of quantum mechanics.

Einstein-Podolsky-Rosen (EPR) steering, that is, an intermediate form of quantum correlations with asymmetry in between entanglement and Bell nonlocality, plays an essential role in quantum information processing, such as one-sided device-independent quantum cryptography tasks. Here we theoretically study the direct generation of EPR steering in ${\chi }^{\left(2\right)}$ nonlinear optical processes with cascaded crystals in a self-phase-locked optical parametric oscillator. Based on a standard linearization method, we analyze different conditions for generating EPR steering in Gaussian states with vanishing or nonzero mean fields, and also show bistable regions for different types of EPR steering. In particular, such system can intrinsically generate asymmetry one-way EPR steering, without introducing any postoperation. Our results suggest that the self-phase-locked optical parametric oscillator is a good candidate to generate different types of EPR steering at will by varying the cavity detunings, photon loss, or input pump power.

The asymmetry of a quantum state relative to a translational group is a central concept in many areas of quantum science and technology. An important and geometrically intuitive measure of translational asymmetry of a state is given by the trace-norm asymmetry, which is defined as the trace norm of the commutator between the state and the generator of the translation group. While trace-norm asymmetry satisfies all the requirements for a bona fide measure of translational asymmetry of a state within the quantum resource theoretical framework, its meaning in terms of laboratory operations is still missing. Here, we first show that the trace-norm asymmetry is equal to the average absolute imaginary part of the weak value of the generator of the translation group optimized over all possible orthonormal bases of the Hilbert space. Hence, it can be estimated via the measurement of weak value combined with a classical optimization in the fashion of quantum variational circuit which may be implemented using the near-term quantum hardware. We then use the link between the trace-norm asymmetry and the nonreal weak value to derive the relation between the trace-norm asymmetry with other basic concepts in quantum statistics. We further obtain trade-off relations for the trace-norm asymmetry and quantum Fisher information, having analogous forms to the Kennard-Weyl-Robertson uncertainty relation.

We introduce a strategy to regulate the quantum entanglement in a dispersive-hybrid system where a qubit is directly coupled to a cavity and a resonator. A dramatic transition takes place by only tuning the squeezing parameters associated with the vibrational mode. As the squeezing amplitude becomes larger, the maximal entanglement abruptly falls to zero at specific squeezing phases. It is also possible to generate entanglement for bipartitions from the qubit-cavity-resonator system after applying this strategy. Entangled qubit-cavity states are created through squeezing, even though there is no direct interaction between them. We also analyze the effect of atomic, optical, and vibrational losses on the quantum entanglement. Finally, we discuss future realizations to implement all these ideas and promote further studies to generalize the concept of monogamy in tripartite systems outside qubit-composite states, in particular, $(2\otimes 2\otimes n)$-dimensional systems.

We study the quantum correlations embedded in open quantum Rabi systems. Specifically, we study how the quantum correlation depends on the coupling strength, number of qubits, and reservoir temperatures. We numerically calculate the quantum correlations of up to three qubits interacting with a single field mode. We find that the embedded quantum correlations exhibit a maximum for a given coupling strength, which depends inversely on the number of subsystems and the reservoir temperature. We explore how this feature affects the performance of a many-qubit Otto heat engine, finding numerical evidence of a direct correspondence between the minimum of the extractable work and the maximum of the embedded quantum correlations in the qubit-cavity bipartition. Furthermore, as we increase the number of qubits, the maximum extractable work is reached at smaller values of the coupling strength. This paper could help design more sophisticated quantum heat engines that rely on many-body systems with embedded correlations as working substances.

Nonadiabatic holonomic quantum computation has received increasing attention due to the merits of both robustness against control errors and high-speed implementation. A crucial step in realizing nonadiabatic holonomic quantum computation is to remove the dynamical phase from the total phase. For this reason, previous schemes of nonadiabatic holonomic quantum computation have to resort to the parallel transport condition, i.e., requiring the instantaneous dynamical phase to be always zero. In this paper, we put forward a strategy to design nonadiabatic holonomic quantum computation, which is based on a commutation relation rather than the parallel transport condition. Instead of requiring the instantaneous dynamical phase to be always zero, the dynamical part of the total phase is separated from the geometric part and then removed by properly choosing evolution parameters. This strategy enhances the flexibility to realize nonadiabatic holonomic quantum computation as the commutation relation is more relaxed than the parallel transport condition. It provides more options for realizing nonadiabatic holonomic quantum computation and hence allows us to optimize realizations such as the evolution time and evolution paths.

In this study, we evaluated a polarization scheme within the framework of the Schwinger multichannel method, using a model potential to describe positron-molecule polarization and correlation. Our aim was to calculate integral and differential cross sections for elastic scattering of positrons by noble gas atoms, hydrogen molecules, and nitrogen molecules. We compared our results with previous theoretical and experimental data, as well as with new results from a parallelized version of our computer codes, which allowed us to push the limits of our current ab initio methodology for small molecules. Our findings show that the model potential can recreate the $s$-wave integral cross sections from our best ab initio calculations, and also helps better understand the scattering process.

Bosons in periodic potentials with very strong local interactions, known as constrained bosons, often exhibit interesting physical behavior. We investigate the ground-state properties of a two-leg Bose-Hubbard ladder by imposing a three-body constraint in one leg and a hard-core constraint in the other. By using the cluster mean-field theory approximation and the density matrix renormalization group method, we show that at unit filling, for strong two-body attraction among the three-body constrained bosons, the system becomes a gapped pair-Mott insulator where all the bosons form strong bound pairs and occupy the leg with the three-body constraint. With increase in hopping strength, this pair-Mott insulator phase undergoes a phase transition to the gapless superfluid phase for equal leg and rung hopping strengths. However, when the rung hopping is stronger compared to the leg hopping, we obtain a crossover to another gapped phase which is called the rung-Mott insulator phase where the bosons prefer to delocalize on the rungs rather than the legs. By moving away from unit filling, the system remains in the superfluid phase except for a small region below the gapped phase where a pair superfluid phase is stabilized in the regime of strong attractive interaction. We further extend our studies by considering the three-body constraint in both the legs and find that the crossover from the gapped to gapped phase does not occur; rather, the system undergoes a transition from a pair-rung-Mott insulator phase to the superfluid phase at unit filling. Moreover, in this case, we find the signature of the pair superfluid phase on either side of this gapped phase.

We continue the study of dispersive quantum shock waves in a one-dimensional Bose gas beyond the mean-field approximation. In a recent work by Simmons et al. [Phys. Rev. Lett. 125, 180401 (2020)], the oscillatory shock wave train developing in this system from an initial localized density bump on a uniform background was interpreted as a result of quantum mechanical self-interference, wherein the interference contrast would diminish with the loss of matter-wave phase coherence. Such loss of coherence, relative to the mean-field Gross-Pitaevskii description, occurs due to either quantum or thermal fluctuations, as well as in the strongly interacting regime. In this work we extend the analysis of dispersive quantum shock waves in this context to other dynamical scenarios. More specifically, the scenarios studied include evolution of a sufficiently high-density bump, known to lead to the so-called “vacuum point” in the mean-field description, and evolution of an initial density dip, known to shed a train of gray solitons in the same mean-field approximation. We study the fate of these nonlinear wave structures in the presence of quantum and thermal fluctuations, as well as at intermediate and strong interactions, and show that both the vacuum point and gray solitons cease to manifest themselves beyond the mean-field approach. On the other hand, we find that a vacuum point can occur in an ideal (noninteracting) Bose gas evolving from a ground state of a localized dimple potential. Due to the ubiquity of dispersive shock waves in nature, our results should provide useful insights and perspectives for a variety of other physical systems known to display nonlinear wave phenomena.

We examine the stability of a one-dimensional electrical transmission line in the simultaneous presence of $\mathcal{P}T$-symmetry and fractionality. The array contains a binary gain/loss distribution ${\gamma }_n$ and a fractional Laplacian characterized by a fractional exponent $\alpha$. For an infinite periodic chain the spectrum is computed in closed form, and its imaginary sector is examined to determine the stable/unstable regions as a function of the gain/loss strength and fractional exponent. In contrast to the nonfractional case where all eigenvalues are complex for any gain/loss, here we observe that a stable region can exist when gain/loss is small and the fractional exponent is below a critical value, $0<\alpha <{\alpha }_{c_1}$. As the fractional exponent is decreased further, the spectrum acquires a gap with two nearly flat bands. We also examined numerically the case of a finite chain of size $N$. Contrary to what happens in the infinite chain, here the stable region lies always above a critical value ${\alpha }_{c_2}<\alpha <1$. An increase in gain/loss or $N$ always reduces the width of this stable region until it disappears completely.

Quantum mechanics may revolutionize many aspects of modern information processing as it promises significant advantages in several fields such as cryptography, computing, and metrology. Quantum cryptography, for instance, allows us to implement protocols which are device independent, i.e., they can be proven secure under fewer assumptions. These protocols rely on using devices producing nonlocal statistics and ideally these statistics would correspond to extremal points of the quantum set in the probability space. However, even in the Clauser-Horne-Shimony-Holt (CHSH) scenario (the simplest nontrivial Bell scenario) we do not have a full understanding of the extremal quantum points. In fact, there are only a couple of analytic families of such points. Our first contribution is to introduce two families of analytical quantum extremal points by providing solutions to two families of Bell functionals. In the second part we focus on developing an analytical criteria for extremality in the CHSH scenario. A well-known Tsirelson-Landau-Masanes criterion only applies to points with uniform marginals, but a generalization has been suggested in a sequence of works by Ishizaka. We combine these conditions into a standalone conjecture, explore their technical details, and discuss their suitability. Based on the understanding acquired, we propose a set of conditions with an elegant mathematical form and an intuitive physical interpretation. Finally, we verify that both sets of conditions give correct predictions on these families of quantum extremal points.

Periodical driving often leads to nontrivial steady states. In the context of open systems, such states may have nonvanishing currents. In this work, we experimentally study the time-discrete open system dynamics of a two-level system where the system-environment coupling is varied periodically. The two-level system is decoded on the transverse polarization and its environment on the frequency degrees of freedom of a single photon. The dynamics is implemented using linear optics. We find that the periodic driving leads to nonvanishing bidirectional information flow between the open system and the environment. We develop a comprehensive theoretical model which explains the experimental findings.

The main goal of this paper is to provide some insight into how promising the Schrödinger-Newton equation would be to explain the emergence of classicality. Based on the similarity of the Newton and Coulomb potentials, we add an electric self-interacting term to the Schrödinger-Newton equation for the hydrogen atom. Our results rule out the possibility that single electrons self-interact through their electromagnetic field. Next, we use the hydrogen atom to get insight into the intrinsic difficulty of testing the Schrödinger-Newton equation itself and conclude that the Planck scale must be approached before sound constraints are established. Although our results cannot be used to rule out the Schrödinger-Newton equation at all, they might be seen as disfavoring it if we base our reasoning on the resemblance between the gravitational and electromagnetic interactions at low energies.

The filter function formalism from quantum control theory is typically used to determine the noise susceptibility of pulse sequences by looking at the overlap between the filter function of the sequence and the noise power spectral density. Importantly, the square modulus of the filter function is used for this method. In this work we show that by using the square modulus one neglects valuable information about the system dynamics. We take advantage of the full filter function by including information about the phase of the perturbation and the resulting rotation axis. By decomposing the filter function with phase preservation before taking the modulus, we are able to consider the contributions to $x, y$, and $z$ rotations separately. Continuously driven systems provide noise protection in the form of dynamical decoupling by canceling low-frequency noise; however, generating control pulses synchronously with an arbitrary driving field is not trivial. Using the decomposed filter function we look at the controllability of a system under arbitrary driving fields, as well as the noise susceptibility, and also relate the filter function to the geometric formalism.

To perform reliable quantum computation, quantum error correction is indispensable. In certain cases, continuous covariance symmetry of the physical system can make exact error correction impossible. In this work we study the approximate error correction and covariance symmetry from the information-theoretic perspective. For general encoding and noise channels, we define a quantity named infidelity to characterize the performance of the approximate quantum error correction and quantify the noncovariance of an encoding channel with respect to a general Lie group from the asymmetry measure of the corresponding Choi state. In particular, when the encoding channel is isometric, we derive a trade-off relation between infidelity and noncovariance. Furthermore, we calculate the average infidelity and noncovariance measure for a type of random code.

Indefinite causal order has found numerous applications in quantum computation, quantum communication, and quantum metrology. Before its usage, the quality of the indefinite causal order needs to be first certified, and the certification should ideally be device independent (DI) to avoid the impact of device imperfections. In this work, we initiate the study of the sequential DI certification of an indefinite causal order. This can be useful in experimental platforms where the generation of an indefinite causal order is difficult. We show that an arbitrary number of sequential DI certifications of an indefinite causal order can be achieved with a quantum switch and also analyze practical requirements for experimental implementations of the certifications. Our work opens the possibility of reusing the resource of an indefinite causal order multiple times in device-independent quantum information processing.

When observing a quantum field via detectors with access to only the mixed states of spatially separated, local regions—a ubiquitous experimental design—the capacity to access the full extent of distributed entanglement can be limited, shrouded by classical correlations. By performing projective measurements of the field external to two detection patches and classically communicating the results, underlying pure states may be identified for which entanglement quantification is clear. In the Gaussian continuous-variable states of the free scalar field vacuum, this protocol uncovers a disparity between the spacelike entanglement established within the field and that which is locally detectable. This discrepancy is found to grow exponentially with the separation between observation regions. The protocol developed herein offers insight and practical guidance for clarifying the unavoidable distortion of quantum field correlations when viewed from the vantage of a pair of local observers.

In this work, we investigate the collision process of antiprotons incidents on atomic hydrogen for energies in the range 0.003–900 keV. We solve the time-dependent Schrödinger equation, in a numerical lattice, for the electron by implementing a Crank-Nicholson implicit method. For the proton and antiproton, a classical description coupled to the quantum electron is proposed to obtain the electron-nuclear dynamics within a semiclassical impact parameter approach. The ionization process is treated by means of an absorbing masking function. We find that the absorbing layer induces an exponential behavior in the ionization process allowing one to determine the asymptotic ionization limit at large times. We assess the validity of the Fermi-Teller criteria for the ionization process at intermediate collision energies. At low collision energies the formation of protonium dominates the ionization process at small impact parameters, while at higher energies larger impact parameters contribute. Excitation probabilities into the $n=2$ energy level of hydrogen are also reported. Finally, the total, electronic, and nuclear stopping cross sections are determined for collision energies above the closing of the protonium formation channel. The nuclear energy loss has a larger contribution than the electronic at low collision energies, as a consequence of the polarization induced by the antiproton on the hydrogen target. Good agreement is found when compared to available experimental data and other theoretical approaches.

This study examines the high-harmonic-generation spectra of hydrogen cyanide, focusing on the contrast between even and odd harmonics and their emissions under two opposite laser-molecule orientations. At lower energies, the emissions are higher when the potential energy of the hydrogen side is lowered, while at higher energies, the emissions are more pronounced when the potential energy of the nitrogen side is reduced. When the laser polarization aligns parallel to the molecular axis, the combined contributions of the $\sigma$ and $\pi$ orbitals result in a two-electron process, leading to varied intensity ratios between even and odd harmonics that a single active electron model cannot adequately explain.

Optical soliton molecules, the self-organized structures in dissipative systems arising from the equilibrium between attractive and repulsive forces, show great importance for revealing soliton interactions and potential applications in optical communications. Here, we numerically and experimentally demonstrate that two orthogonally polarized components with mirrored profiles can form vector soliton molecules in single-mode fiber lasers containing a section of polarization-maintaining fiber. Due to the dynamic balance between fiber birefringence and chromatic dispersion, the temporal dislocation between two components changes along the cavity while reaching the self-consistent state per round trip. The unequal coupling for orthogonally polarized modes at the input interface of the polarization-maintaining fiber dominates the asymmetric property of subpulses for each component. Such soliton molecules are observed both in erbium-doped and ytterbium-doped fiber lasers, indicating the generality of the vector asymmetric structures. Our work reveals soliton molecules with additional degrees of freedom in polarization and intensity, expanding the understanding of the asymmetric mode interactions in fiber lasers.